jumlah tak hingha 9+3+1....adalah

9+3= 12+1 = 13,

semoga membantu

r = u2/u1 <=> 3/9 = 1/3
Jumlah tak  hingga = a / (1-r)
                                  = 9 / (1-1/3)
                                  = 9 / (2/3)
                                  = 27/2 = 13,5